Optical transmitters for long haul applications at multi-gigabit rates are usually implemented by use of a Lithium Niobate Mach Zehnder (MZ) modulator to gate continuous wave (CW) laser light. The component is well established and applicable but has some detrimental drift and ageing characteristics that require external control to maintain peak performance.
Conventionally these control circuits are arranged to optimise optical extinction ratio and maintain a maximum modulation depth. Typically, drive level and bias voltage are adjusted separately and the effect monitored by sampling the average transmitter output signal power. Conventionally, the AC coupled input signal drive voltage summed with a DC bias is arranged to exercise the modulator from its peak output light level to its minimum output light level in a closely linear fashion. Such operation will optically re-create a representation of the input signal be it return-to-zero (RZ) or non return-to-zero (NRZ) in format. It is conventionally assumed that the control methods should optimise extinction ratio (ER) and achieve a symmetrical output waveform of 50% duty cycle, and that this will lead to ideal transmission through the system.
However, in practice long haul transmission systems suffer from non-linear distortion, dispersion and self-phase modulation (SPM) effects.
Dispersion effects can in theory be addressed by adjusting the dispersion settings before and after the effects occur (i.e. pre- and post-transmission). There will be a particular amount of pre- and post- dispersion setting adjustment required to obtain best transmission for a given signal. This may be done by passing the signal from each channel through a dispersive element at both the transmission and receiver ends. However, it is not operationally or economically practical to fine tune the launch and receive dispersion values of every channel in a WDM signal in this way.
SPM impairments are often evident on the rising and falling edges of a return-to-zero (RZ) format, whereas SPM is only evident on the ‘1-0’ or ‘0-1’ transitions of non-return-to-zero (NRZ) formats.
It has been found that a clock chirped RZ (CRZ) format may be used to alleviate some of the unwelcome effects of SPM and dispersion. In particular, the chirp may be used to partially mitigate dispersion and the SPM effects at the transition edges. For these reasons, a chirped RZ (CRZ) format has been developed for use in long haul transmission systems.
Chirp is typically added to the RZ signal by a Lithium Niobate phase modulator placed after the data modulator. A clock driver set to be synchronous with the data signal drives this phase modulator.
Though the combination of a CRZ format and appropriately chosen dispersive fibres described above has some success in combating SPM, non-linear distortion, and dispersion in the transmission fibre it suffers from a number of drawbacks. In particular, this solution is both complex and expensive. For example, the clock driver and phase modulator are costly and must be calibrated and tested if they are to be effective.